A gas turbine engine includes a compression section, a combustion section, and a turbine section. A flow path for the gases extends axially through the engine. The compression section has a rotor assembly that includes an array of rotor blades that are angled with respect to the approaching gas flow. The compression section also has a stator assembly that includes an array of stator vanes. Duct walls extend circumferentially with respect to the flow path to form the boundaries for the working medium gases in the fan section.
As the rotor assembly rotates, the blades increase the pressure of the gases. The rotor blades also increase the velocity of the gases and direct the flow of gases from the engine axial direction to the blade rotation direction. The gases are then flowed past the rotor blade array to the stator vane cascade, which redirects the flow of gases to the axial direction. By reorienting the flow in this manner, the stator vane cascade increases the recovery of the flow energy of the gases into thrust.
As the working medium gases travel along the engine flow path, the gases are pressurized in the rotating compression section, which causes the gas temperature to rise. The hot, pressurized gases are burned with fuel in the combustion section to add energy to the gases. The gases are then expanded through the rotating turbine section to produce useful work and for pressurizing the gases in the fan and compression sections.
The gas flow through the engine generates acoustic energy or noise. The main sources of noise in a gas turbine engine exhaust are combustion noise, turbulent noise, and fan noise. Combustion noise originates within the combustion chamber of the gas turbine. Turbulent noise includes all the dynamic instabilities resulting from the high velocity exhaust stream such as the jet noise resulting from the localized flow separation at boundary discontinuities, or periodic high velocity flow impact on exhaust duct structural boundaries. Fan noise results from the periodic passage of the individual turbine blades which creates pressure pulsations that propagate downstream in the exhaust duct in the same way that inlet fan noise is generated and propagates out the gas turbine inlet.
U.S. Pat. No. 4,244,441 to Tolmon describes a broad band acoustic attenuator for attenuating gas turbine engine noise using a plurality of axially extending, open-ended, perforated cylinders concentrically arranged within the exhaust duct of the gas turbine engine for attenuating noise therefrom without imposing significant back pressure penalties.
U.S. Pat. No. 6,035,964 to Lange describes a combined device for positioning between the outlet of a gas turbine and a steam generator. The combined device acts as a sound-absorber and as a diffuser and is designated a gas turbine muffler. The gas turbine muffler has an inner zone, which widens out in the flow direction at a relatively large angle. Deflector elements arranged in this inner zone delineate diffuser channels that are located between adjacent deflector elements. The diffuser channels widen out in each case at a significantly smaller acute angle of less than seven degrees. In addition to decelerating the stream of gas and hence, in addition, increasing the pressure, the narrow diffuser channels also bring about sound-absorption by reducing the regions of turbulence, making the stream more uniform, and aligning the stream.
The silencing of gas turbine installations in industrial environments may include attenuating the noise from the atmospheric inlet and the exhaust openings. The sound from the gas turbine enclosure, auxiliaries, and driven equipment also may be considered in the silencing approach. Some degree of attenuation can be attained by natural factors, such as divergence, directivity, air absorption, and duct configurations. Further sound control, however, is typically obtained through the use of adequate silencer or sound suppressor.
A sound suppressor normally contains one or more passages which are lined with sound absorbing material, incorporated in structures that protect them from erosion from air flow. To attenuate low frequency sounds, thick baffles widely spaced are typically used; whereas for high frequency sounds, sound absorption is obtained by using thin baffles closely spaced. Increased attenuation may be achieved by incorporating bends or elbows into the sound suppressor.
Some conventional exhaust sound suppressors address attenuation of low frequency noise by attempting to disrupt in-duct acoustical standing waves by using pipes installed normal to the flow direction, or plates installed parallel to the flow direction. However, these approaches may introduce flow effects, e.g. wake shedding, that cause additional acoustical standing waves where none had existed before.